1. Field of the Invention
The invention relates to fiber placement machines that use multiple strands or tows (or rovings) of fiber that are pulled from a creel assembly and placed on a surface of a workpiece.
2. State of the Art
Filament winding was one of the early attempts at automating the laying down of composites on forms. Early filament winding machines consisted of a spindle axis which rotated a mandrel, whose outer shape and dimensions are the same as the shape and internal dimensions of the part to be wound. A carriage axis moved in a direction parallel to the spindle. A cross slide was mounted on the carriage providing movement perpendicular to the spindle axis and the carriage axis. An eye, or circular opening, was mounted to the end of the cross slide and was rotatable about an axis parallel to the cross slide axis. Tows made up of filaments of fiber glass, Kevlar.TM. polyamide material, or carbon were transported from spools located in a creel, through a cup of liquid resin and wound under tension onto the mandrel.
In filament winding, the winding is started by attaching the roving to the mandrel and rotating the mandrel while moving the carriage longitudinally to the axis of the mandrel. The angle of winding is determined by the relative speed of rotation of the mandrel and the linear motion of the carriage. The angle has to be maintained during lay down in one direction of motion of the carriage slide. When the carriage reaches the opposite end of the mandrel, it reverses direction. Two means are used to keep the tows from sliding off the mandrel when the longitudinal direction is reversed. The first method is used for structures having domed ends. In this case, the mandrel is rotated through a relatively large angle as the tow is placed on the dome. The fiber path essentially runs tangent to the mandrel support rod. As the tow passes the tangency point, the carriage is reversed. As layers are added to the lay down, the wall thickness of the domed ends increases to a maximum at the tangent point. The second method uses a pin ring made up of several small pins located around the periphery of the mandrel ends. The tows are held in the spaces between the pins.
Early filament winding machines controlled the angle through a mechanical cam or gearing arrangement. As computer controls became viable, the movements of the various axes were controlled by a computer program, which runs on a computer numerical control (CNC).
Filament winding is limited to shapes that are nearly cylindrical with the filaments following geodesic paths. A geodesic path is a path of a material over a form, said path being selected to reduce most structural stresses to tensions that allow the material to lie in place on the form congenially without tending to move from its placid position.
Tape laying is a more recent attempt to automate the lay down of composites on mandrels. In this process, prepreg composite tapes are fed from a reel to a delivery roller and compacted onto the mandrel or onto previous layers of prepreg tape. The tape usually has a paper or similar backing to keep the roll of tape from adhering to itself. A limitation of this process is that the tape path must follow a geodesic path in order to keep from buckling. The tape is cut at the end of each pass. The tape remains in place after cutting by means of the tackiness of the preimpregnation. In most cases, tape laying forms or molds are stationary and nearly flat. Tape laying is limited to geodesic paths across surfaces.
Fiber placement, although related to both filament winding and tape laying processes, is more versatile than either of them because it may be used to lay down tows of fiber on flat or symmetrical shapes as well as on complex compound contours that previously required manual darting and fitting of the material onto the shape. It uses a compaction roller like tape laying, but uses material in the form of prepreg tows instead of tape. It forms its own tape or band from the tows. The reason that fiber placement can move on non-geodesic paths is that each tow can feed at a different rate, allowing the outside tow on a curve to feed more rapidly than the inside tow. The tows between the inside and outside tows feed at progressively faster rates from the inside to the outside, thus permitting the steering of the fiber into curved or non-geodesic paths.
Previous fiber placement machines provided no means of processing the tows into flat ribbons or of controlling the temperature of the tows being processed. Flattening of the tows was performed solely by the compaction roller pressure as the tows were compacted against the mandrel.
Tows for fiber placement may be impregnated with either thermosetting resins or thermoplastic resins. Thermosetting resins cure over a given period of time, depending on the temperature of their environment. Elevating the temperature speeds the rate of cure, while lowering the temperature decreases rate of cure, even to the extent of stopping the cure altogether at lower temperatures. Most thermosetting prepreg tows will gradually cure at room temperature. Curing renders the tows stiff and brittle, making fiber placement difficult or impossible. On the other hand, thermoplastic resins do not cure at any temperature but soften at elevated temperatures and stiffen at lower temperature to the point of becoming solid. Some of these thermoplastic resins can repeatedly be heated to soften and cooled to stiffen. Contrary to this, thermosetting resins once reheated above their softening point will degrade and carbonize rather than re-soften.
Prepreg tows have the property of tackiness, which allows separate tows to adhere to each other or to a mandrel, thus providing a means of building up various solid shapes by layers. Tackiness of tows is affected by the temperature of the tows, increasing with increased temperature, when using thermoplastic resins or before thermosetting resins become completely cured. High tackiness, during processing, is undesirable before the tows reach the compaction roller, because tacky resins adhere to the components of the delivery head with which they contact. Build up of resin on the delivery heads detracts from the functions of the delivery heads.
U.S. Pat. No. 4,943,338 discloses a fiber placement machine that has a delivery head for controlling fiber tows coming from a supply creel to a work lay down zone. This machine travels on an overhead gantry structure for positioning and delivering the tows to the workpiece with the help of a computer numerical control (CNC) for controlling the actual spatial positioning and manipulation of the head.